This invention relates to a thermosensitive recording material and, more particularly, to a thermosensitive recording material improved in thermal response and image retention.
Thermosensitive recording materials are generally composed of a support and, provided thereon, a thermosensitive recording layer containing as major constituents an ordinarily colorless or slightly colored dye precursor and an electron receptive developer. Upon being heated by means of a thermal head, thermal pen, or laser beam, the major constituents instantly reacts with each other to form a recorded image, as disclosed by Japanese Patent Examined Publication Nos. 4,160/68 and 14,039/70. Because of the advantages of comparatively simple design and easy maintainance, the recording devices employing such thermosensitive materials are being used in a wide field including recording instruments for measurements, facsimiles, printers, terminal devices for computers, labels, and automatic vending machines for railroad tickets and the like.
The fundamental performance characteristics required for the thermosensitive recording material include sufficient density of the color image, high sensitivity, and freedom from the deterioration in color image with time. Furthermore, the recent strive for the speed-up of impression recording demands the development of both the high speed recording device itself and the corresponding recording material which answers to the required speed. For this purpose, it is necessary to develop a recording material of a sufficient thermal response capable of producing a high-density color image at a low thermal energy. As the use of thermosensitive recording materials becomes popular, the fat resistance of the recording material is also required to keep the recorded image from deterioration due to the contact with oily substances such as fatty matters contained in hairdressing preparations, hand creams, or skin secretions. Thus, the advent of a thermosensitive recording material satisfactory in both the thermal response and the image retention is requested.
It has, heretofore, been known to incorporate various additives into the thermosensitive recording layer in order to produce a high-density color image, such as, for example, urea, phthalic anhydride, acetanilide, or the like, as disclosed in Japanese Patent Examined Publication No. 4,160/78; natural and synthetic waxes such as beeswax, carnauba wax, or paraffin wax, as disclosed in Japanese Patent Examined Publication No. 19,231/73; and salicylic acid, monobenzyl phthalate, or the like, as disclosed in Japanese Patent Examined Publication No. 17,748/74. Japanese Patent Unexamined Publication (Lain-open) No. 119,893/83 discloses the addition of bis(4-hydroxyphenyl)sulfone as developer to improve the image retention. These additives, however, are found to be unable to act satisfactorily, because of their undesirable effects on the characteristics of thermosensitive recording materials, such as insufficient thermal response to produce a satisfactory image density, or insufficient image retention, the image becoming deteriorated with time even though an image of sufficient density is produced.